Question

NADH and FADH2 are often considered “energy currency” in cellular metabolism because they are generated
directly from Glycolysis and the TCA cycle. Using the following half-cell potentials, calculate the Gibbs Free Energy
that can be derived from the oxidation of NADH and FADH2. Use the Faraday Constant at 96,500 J/V mol.
FAD + 2H+ +2 e- = FADH2 Eo = -0.219V
NAD+ + H+ + 2 e- = NADH Eo = -0.320V
a. 21.3 and 30.8 kJ/mol
b. -21.3 and -30.8 kJ/mol
c. 42.3 and 61.8 kJ/mol
d. -42.3 and -61.8 kJ/mol

Answer 1

#1. Given, FAD + 2H⁺ + 2e ------> FADH₂         ; E⁰ = - 0.219V          - reaction 1

That is, reduction (reduction = gain of electrons) of FAD reduces redox potential by 0.219V energy.

# We have to calculate, free energy change for oxidation of FADH₂.

So, write the reaction for oxidation of FADH₂ by reversing reaction 1-

            FADH₂ ------> FAD + 2H⁺ + 2e               ; E⁰ = + 0.219V         - reaction 1”

When a reaction is reversed, the sign of its redox potential becomes opposite to that of the original reaction.

Now, using dG⁰ = - nF (dE⁰)

            Where, n = number of electrons transferred

                        F = faraday’s constant

                        dE⁰ = redox potential

Now,

dG⁰ for FAD oxidation,

                        dG⁰ = - 2 x (96,500 J/V mol) x (+ 0.219 V)

                                    = - 42267 J/mol

    = - 42.267 kJ mol^-1

#2. Given, NAD⁺ + H⁺ + 2e ------> NADH        ; E⁰ = - 0.320V          - reaction 2

That is, reduction (reduction = gain of electrons) of NAD⁺ lowers the redox potential by 0.320 V.

# We have to calculate, free energy change for oxidation of NADH.

So, write the reaction for oxidation of NADH by reversing reaction 2-

            NADH ------> NAD⁺ + H⁺ + 2e                 ; E⁰ = + 0.320V

When a reaction is reversed, the sign of its redox potential becomes opposite to that of the original reaction.

Now,

dG⁰ for NADH oxidation-

                        dG⁰ = - 2 x (96,500 J/V mol) x (+ 0.320 V)

                                    = - 61760 J/mol

    = - 61.76 kJ mol^-1

So, correct option is – D.

# The –ve sign of dG⁰ indicates that energy is being released during the process of oxidation of FADH₂ and NADH.